1. Field of the Invention
The present invention relates to a dielectric filter, a dielectric duplexer, and a communication device.
2. Description of the Related Art
A known dielectric filter in which a plurality of dielectric resonators are provided in a dielectric block is shown in FIG. 20. The dielectric filter 200 is formed in a dielectric block 201 having a generally parallelepiped shape. A pair of resonator holes 202a and 202b are formed in the dielectric block, each hole extending between opposing surfaces 200a and 200b of the dielectric block. The resonator holes 202a and 202b have large-diameter hole portions 222a and 222b, and small-diameter hole portions 223a and 223b communicating with the large-diameter hole portions 222a and 222b, respectively.
As best shown in FIG. 21, the end walls 224a and 224b of the large-diameter hole portions 222a and 222b and end walls 225a and 225b of the small-diameter hole portions 223a and 223b are formed in a common plane. The axes of the small diameter hole portions 223a and 223b are displaced from those of the large diameter hole portions 222a and 222b with the result that relatively small communication areas b are formed between the respective large and small diameter hole portions.
An outer conductor 204 is formed on five of the six outer surfaces of the dielectric block. The front surface 200a is not plated. A pair of input/output electrodes 205 are formed on the outer surface of the dielectric block 201 and are spaced from the outer conductor 204 so as to be electrically isolated therefrom. Inner conductors 203 are formed on the entire inner surface of each of the resonator holes 202a and 202b. The end of the inner conductors 203 located at the front surface 200a of the dielectric block is electrically open (i.e., spaced from, and thereby isolated from, the outer conductor 204). The end of the inner conductors 203 located at the rear surface 200b is short-circuited (physically connected) to the outer conductor 204.
The outer conductor 204 and inner conductors 203 are typically formed on the dielectric block 201 by wet plating. However, with wet plating, the plating liquid in the vicinity of a surface to be plated must be circulated so that new plating liquid is constantly supplied to the surface. To this end, plating liquid is typically stirred or the workplace is moved in the plating liquid to promote the circulation of the plating liquid.
As best shown in FIG. 21, the connection portions b between the large and small diameter portions are narrow. This results in poor penetration of the plating liquid through the resonator holes 202a and 202b, and thus results in a smaller supply of new plating liquid and insufficient plating. With this arrangement, therefore, it is difficult to provide the desired film thickness for the inner conductor 203 to be formed on the inner surface of the resonator holes 202a and 202b. 
An object of the present invention is to provide a dielectric filter, dielectric duplexer, and communication device, which allow the formation of an inner conductor on the inner surfaces of resonator holes with sufficient thickness and stability.
To this end, according to a first aspect of the present invention, there is provided a dielectric filter includes a dielectric block having a plurality of resonator holes therein, an inner conductor formed on the inner surface of each of the resonator holes, and an outer conductor formed on the outer surface of the dielectric block. At least one of the resonator holes comprises a large-diameter hole portion and a small-diameter hole portion communicating with the large-diameter hole portion. The axis of the large-diameter hole portion and the axis of the small-diameter hole portion are displaced from each other so that the at least one of the resonator holes has a bent shape. The large-diameter hole portion and the small-diameter hole portion overlap each other along their respective axial directions.
With this arrangement, the connection portion of the large-diameter hole portion and the small-diameter hole portion is larger in cross section (as measured along a plane lying perpendicular to the main direction of flow of plating liquid through the connection portion) than the connection portion of the known resonator hole, thereby improving the passage of plating liquid through the resonator hole. As a result, it is easier to ensure that the film thickness of the inner conductor of the large-diameter hole portion and the small-diameter hole portion is at desired levels, thus allowing an increase of the Q-value of the resonator. This makes it possible to broaden the passband of the dielectric filter and to facilitate the achievement of the small-sized dielectric filter having an acute attenuation characteristic and high performance.
Preferably, the dielectric filter includes at least two bent resonator holes located adjacent one another and the interaxial distance between the small-diameter hole portions of two adjacent resonator holes is greater than, equal to, or smaller than the interaxial distance between the large-diameter hole portions thereof.
According to a second aspect of the present invention, there is provided a dielectric duplexer. The dielectric duplexer which includes the dielectric filter according to the first aspect of the present invention.
According to a third aspect of the present invention, there is provided a communication device which includes a dielectric duplexer according to the second aspect of the present invention.
Since the dielectric duplexer and the communication device according to the present invention include the dielectric filter having the above-mentioned features, they can provide improved electric characteristics similar to those of the dielectric filter of the present invention.